The invention is generally related to biomedical devices for sensing physiological parameters and, more particularly, to catheters having temperature sensing devices.
The heart beat in a healthy human is controlled by the sinoatrial node ("S-A node") located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node ("A-V node") which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of or damage to the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as cardiac arrhythmia.
Electrophysiological ablation is a procedure often successful in terminating cardiac arrhythmia. This procedure involves applying sufficient energy to the interfering tissue to ablate that tissue thus removing the irregular signal pathway. However, before an ablation procedure can be carried out, the interfering tissue must first be located.
One location technique involves an electrophysiological mapping procedure whereby the electrical signals emanating from the conductive endocardial tissues are systematically monitored and a map is created of those signals. By analyzing that map, the interfering electrical pathway can be identified. A conventional method for mapping the electrical signals from conductive heart tissue is to percutaneously introduce an electrophysiology ("EP") catheter having mapping electrodes mounted on its distal extremity. The catheter is maneuvered to place those electrodes in contact with or in close proximity to the endocardium of the patient's heart. By monitoring the electrical signals at the endocardium, aberrant conductive tissue sites responsible for the arrhythmia can be pinpointed.
Once the origination point for the arrhythmia is located in the tissue, the physician may use an ablation procedure to destroy the tissue causing the arrhythmia an attempt to remove the electrical signal irregularities and restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels.
The distal end of an EP catheter may include the mapping electrodes as well as an ablation device mounted near the tip for performing the ablation procedure. One type of ablation device includes an ablation electrode that emits radio frequency ("RF") energy to heat the target tissue to a temperature high enough to cause ablation of that tissue. Other types of ablation devices may be used and in the following disclosure, an ultrasonic device is disclosed.
As the ablation procedure progresses, heat is generated and the surrounding blood is exposed to this heat. At approximately 90.degree.-100.degree. C., charring and boiling of the blood take place. Charring is particularly troublesome at the surface of the ablation device because emboli may form on the surface of the device to an extent that the catheter must be removed and cleaned before the procedure can continue. Furthermore, in RF ablation procedures, charring can cause a substantial increase in the impedance and a corresponding decrease in the power delivery to the tissue. Too great a rise in impedance can result in sparking and thrombus formation within the heart, both of which are undesirable.
Because part of the ablation transducer is in contact with the blood in the heart, blood boiling, emboli development, and clotting can result if the surface temperature of the transducer exceeds 90.degree.-100.degree. C. If this occurs, the ablation procedure must be stopped regardless of whether the entire ablation procedure has been completed. The catheter must then be removed from the patient, the attached necrotic tissue removed, and the catheter reinserted into the patient. Such cleaning processes require extra time and unduly prolong the ablation procedure. To avoid such undesirable circumstances, a temperature sensor may be incorporated at the distal end of the catheter to monitor and maintain a selected temperature during ablation. The ablation process can then be controlled so that the temperature is not allowed to increase above a predetermined level.
Temperature sensors have been incorporated in catheters for some time. One of these temperature sensors comprises a thermocouple having elongated sensing joined together at their distal ends to form the thermocouple and their proximal ends connected to a connector mounted in the catheter. External equipment may be connected to the connector of the catheter to receive the temperature signals of the thermocouple for processing to determine the temperature sensed. The thermocouple is typically disposed at the distal end of the catheter.
Thermocouples use two leads formed of dissimilar materials, for example one lead formed of constantan and the other lead formed of copper, that are joined at one end to form a thermocouple junction. The thermocouple junction produces a voltage representative of the temperature at the junction and that voltage varies as the junction is exposed to temperature changes. The proximal ends of the leads of dissimilar materials are commonly directed within the catheter to the connector and are connected to connecting devices, such as pins, of that connector. The connector is typically formed with a plurality of metallic connecting devices to which the thermocouple leads and other leads are soldered. If those connecting devices are composed of a material dissimilar to a material of the thermocouple leads, for example copper, then when the constantan thermocouple lead is connected to the copper connecting device, a second thermocouple junction will be formed at that connection point. This second thermocouple junction will also produce a voltage dependent on the temperature of that particular junction and because it is part of the electrical circuit of the distal end thermocouple, this additional voltage will combine with the distal end thermocouple signal resulting in reduced accuracy of the temperature indication of the distal tip of the catheter.
While custom connectors and interconnection cables may be built having certain pins formed of constantan and some of copper, for example, so that thermocouples at the connector are not formed, such devices can be relatively expensive. It would be desirable to be able to use standard, less expensive devices.
Hence, those skilled in the art have recognized the need for a temperature sensing system that compensates for thermocouple effects produced at the connection points in the catheter handle to produce a more accurate temperature indication. A need has also been recognized for such a system that does not significantly increase the expense of the catheter yet provides increased accuracy. The present invention fulfills these needs and others.